In a conventional technique, natural light emitted from a backlight enters liquid crystal cells as it is. Recently, upsizing and high definition of liquid crystal displays have required an increase in backlight brightness. In addition, a number of techniques are beginning to be used for polarization of light from backlights.
In a backside of a liquid crystal cell, for example, the high-brightness polarizing plate with which a polarizing plate and a brightness enhancement film are adhered together is prepared. A brightness enhancement film shows a characteristic that reflects linearly polarized light with a predetermined polarization axis, or circularly polarized light with a predetermined direction, and that transmits other light, when natural light by back lights of a liquid crystal display or by reflection from a back-side etc., comes in. The polarizing plate, which is obtained by laminating a brightness enhancement film to a polarizing plate, thus does not transmit light without the predetermined polarization state and reflects it, while obtaining transmitted light with the predetermined polarization state by accepting a light from light sources, such as a backlight. This polarizing plate makes the light reflected by the brightness enhancement film further reversed through the reflective layer prepared in the backside and forces the light re-enter into the brightness enhancement film, and increases the quantity of the transmitted light through the brightness enhancement film by transmitting a part or all of the light as light with the predetermined polarization state. The polarizing plate simultaneously supplies polarized light that is difficult to be absorbed in a polarizer, and increases the quantity of the light usable for a liquid crystal picture display etc., and as a result luminosity may be improved. That is, in the case where the light enters through a polarizer from backside of a liquid crystal cell by the back light etc. without using a brightness enhancement film, most of the light, with a polarization direction different from the polarization axis of a polarizer, is absorbed by the polarizer, and does not transmit through the polarizer. This means that although influenced with the characteristics of the polarizer used, about 50 percent of light is absorbed by the polarizer, the quantity of the light usable for a liquid crystal picture display etc. decreases so much, and a resulting picture displayed becomes dark. A brightness enhancement film does not enter the light with the polarizing direction absorbed by the polarizer into the polarizer but reflects the light once by the brightness enhancement film, and further makes the light reversed through the reflective layer etc. prepared in the backside to re-enter the light into the brightness enhancement film. By this above-mentioned repeated operation, only when the polarization direction of the light reflected and reversed between the both becomes to have the polarization direction which may pass a polarizer, the brightness enhancement film transmits the light to supply it to the polarizer. As a result, the light from a backlight may be efficiently used for the display of the picture of a liquid crystal display to obtain a bright screen.
The suitable films are used as the above-mentioned brightness enhancement film. Namely, multilayer thin film of a dielectric substance; a laminated film that has the characteristics of transmitting a linearly polarized light with a predetermined polarizing axis, and of reflecting other light, such as the multilayer laminated film of the thin film having a different refractive-index anisotropy; an aligned film of cholesteric liquid-crystal polymer; a film that has the characteristics of reflecting a circularly polarized light with either left-handed or right-handed rotation and transmitting other light, such as a film on which the aligned cholesteric liquid crystal layer is supported etc. may be mentioned.
Therefore, in the brightness enhancement film of a type that transmits a linearly polarized light having the above-mentioned predetermined polarization axis, by arranging the polarization axis of the transmitted light and entering the light into a polarizing plate as it is, the absorption loss by the polarizing plate is controlled and the polarized light can be transmitted efficiently. On the other hand, in the brightness enhancement film of a type that transmits a circularly polarized light as a cholesteric liquid-crystal layer, the light may be entered into a polarizer as it is, but it is desirable to enter the light into a polarizer after changing the circularly polarized light to a linearly polarized light through a retardation plate, taking control an absorption loss into consideration. In addition, a circularly polarized light is convertible into a linearly polarized light using a quarter wavelength plate as the retardation plate.
If the light guide plate itself has a prism structure and a prism-type condensing sheet or the like is used together, polarized light can be emitted from a backlight, through it is slight. In such a case, the polarization performance can be 5% or more, preferably 10% or more, further preferably 15% or more, and a direction of the emitted light does not have to be in the normal direction of the backlight face. The polarization performance is represented by the formula: polarization performance=(maximum brightness−minimum brightness)/(maximum brightness+minimum brightness). The polarization performance is determined by measuring, through a Glan-Thomson prism, changes in brightness (maximum brightness and minimum brightness) of the emitted light from the backlight in the direction of the polarization axis.
In the case that such brightness enhancement films are used, there has been a problem of the amount of color shift. A various kinds of methods have been proposed to reduce the amount of color shift (for example, see Japanese Patent Application Laid-Open (JP-A) Nos. 11-248941, 11-248942, 11-64840, and 11-64841). In these literatures, a reduction in the amount of color shift in the whole of a liquid crystal display is investigated. In JP-A Nos. 11-248941 and 11-248942, a reduction in the amount of color shift of a brightness enhancement film is investigated. In JP-A Nos. 11-64840 and 11-64841, a combination of a brightness enhancement film and a liquid crystal panel is investigated to reduce the amount of color shift. Conventionally, however, the amount of color shift is not sufficiently reduced in the high-brightness polarizing plate that is a laminate of a polarizing plate and a brightness enhancement film.
It is also known that a composite film of an interference multilayer laminate and a stretched polyvinyl alcohol-based film is impregnated with iodine to form a high-brightness polarizing plate of a composite of a brightness enhancement film and a polarizer (for example, see, Japanese Patent Application National Publication (Laid-Open) No. 09-507308, page 12). Such the high-brightness polarizing plate can reduce the amount of color shift to some extent but can have serious unevenness in dyeing of the polarizer, therefore it cannot be applied to image viewing displays such as liquid crystal displays. The polyvinyl alcohol-based film is stretched to 3 times or more (4 times or more or 5 times or more) and controlled to have a water content of 10% or less. In the process of impregnating the resulting film with iodine, the speed of iodine dyeing can vary with variations in the alignment state of the polyvinyl alcohol-based film in the width and variations in the thickness directions and variations in the degree of crystallinity in the width direction, so that thick parts can significantly tend to be deeply dyed and thin parts less deeply dyed. Thus, uneven dyeing can occur in the polarizer, and in-plane unevenness can lead to an insufficient reduction in brightness when black viewing is displayed. It is practically difficult to apply such a composite film to liquid crystal displays or the like.